Rotary mechanical gap type radial face seals are used for effecting a seal between relatively rotatable members such as a shaft and a housing by controlling leakage of the fluid from one region of high pressure to a second region of lower pressure.
Such gap type seals are typically formed with two sealing members. One of the members is fixed so that no movement occurs axially relative to the shaft, and this is referred to as a fixed seal member or, in this case, mating ring. The other element is movable axially along the shaft and is sometimes referred to as a floating element or in this particular case, the sealing ring. The elements each include sealing faces which are located in opposed relationship to each other. The sealing faces are such that, in response to fluid pressure, spring pressure, or both, a sealing relationship will be obtained between them to prevent leakage out along the shaft.
During operation the relatively rotatable sealing members are kept from touching one another during the operation of the seal. This characteristic makes them ideal for very high speeds, since there is no appreciable wear of the sealing members and hence no appreciable destructive heat produced by their relative rotation.
To keep the sealing members from touching one another under design operating conditions, a fluid pressure is created between the confronting seal faces. When this pressure exceeds the pressure (typically called spring pressure) tending to bring the seal faces together the seal faces are separated. The degree of separation is controlled by the action of the fluid as it passes between the faces to the low pressure side of the seal. At startup of the equipment in which the seal is installed, the design fluid pressure is not available. Accordingly, the seal faces are in contact while the pressure in the seal chamber is building up to the design pressure. Such contact, even if brief, may be sufficient to create a heat and wear condition at the faces sufficient to destroy the seal.
One known gap seal is disclosed in James F. Gardner U.S. Pat. Nos. 3,499,653 and 3,804,424. The seal disclosed in these patents is a mechanical end face seal which operates with a gap between the opposed radial sealing faces of the sealing rings to permit controlled leakage. These rings have flat, radially extending surfaces which sealingly engage one another. Shallow spiral grooves are formed in the outer periphery of one of the relatively rotatable seal members, preferably the stationary one, to create a pump, the direction of the spiral being such that the fluid to be sealed is forced between the seal members to separate and lubricate them at start-up. The spiral grooves, however, are effective in only one direction of relative rotation so that the seal is directional and may be objected to for that reason.
A similar known arrangement uses T-shaped grooves instead of spiral grooves. The function is quite similar. The grooves generate pressure to force the faces axially apart.
There are problems with known gap seal designs. For instance, the dimensions of these recesses are critical and difficult to manufacture because of the tight tolerances that are required. Also, any contaminant in the fluid has a significant detrimental effect on the performance of the seal. Accordingly, the hydrodynamic performance range is limited because of the fixed geometry in this structure.
The environment of the present invention is similar to that of then known constructions. Accordingly, reference will be had to one known construction.
Those skilled in the art will recognize the environment of gap seals used today is typically far more sophisticated than the environment described below. The following description is, however, intended simply to illustrate an example of the type of seal improved by the present invention.
FIG. 1 shows a gap seal having a rotor or mating ring 10 mounted on a shaft 11 and having a substantially radially disposed sealing surface 12 which has been appropriately lapped to be perfectly flat and smooth. The mating ring 10 is preferably made in the form of a washer which is finished and lapped independently of shaft 11 and is then assembled with respect to said shaft, in a manner so as to be rotatable therewith. Adjacent mating ring 10 and surrounding shaft 11 is a sealing washer or member 13 having a sealing surface 14 adjacent to and confronting sealing surface 12 on the mating ring 10. The sealing washer is sometimes called the sealing ring; that parlance will be adopted hereinafter. Sealing ring 13 is formed with an axially extending sleeve 15 which fits into an appropriate opening 16 in a housing 17 through which shaft 11 extends. The opening 16 is enlarged at 18 to form a seal chamber in which the mating ring 10 and sealing ring 13 may operate.
Seal chamber 18 is filled with a fluid, either a gas or a liquid, as the case may be, at any desired pressure above atmospheric. The pressure in opening 16, on the other hand, may be atmospheric pressure so that the fluid in seal chamber 18 tends to move radially inwardly between surfaces 12 and 14 and into the space between the sleeve 15 and shaft 11, to the opening 16. A seal of any suitable character, such as an O-ring 19, supports sleeve 15 and sealing ring 13 resiliently in opening 16 to allow said sealing ring to move axially in opening 16, as well as radially, to a limited extent. A very light spring 20 may be retained between sealing ring 13 and the radial wall 21 of chamber 18 to urge sealing ring 13 against mating ring 10 when there is no pressure in chamber 18.
Sometimes the sealing surface 14 is made slightly convex by a lapping operation to provide a wedge-shaped space at the radially outer regions of sealing ring 13 to initiate and maintain the separation of the surfaces 12 and 14 under operating conditions. When this is done, the actual separation at the low pressure side of surface 14 is very small so that the separation shown in FIG. 1 is greatly exaggerated for purposes of illustration. The curvature of the surface 14 is likewise considerably exaggerated.
In theory, under operating pressures, sealing ring 13 will be pushed away from surface 12 a predetermined distance and will then maintain that distance or separation regardless of axial or radial movements of the mating ring 10, the sealing ring 13 being compelled to follow such movements by the pressure effect of the fluid being sealed. This action is such that should any external forces be present tending to reduce the gap between surfaces 12 and 14, the forces of the fluid upon the movable sealing ring 13 will counter such external force and move sealing ring 13 to the right, as shown in FIG. 1, until the designed gap is created. Similarly, should the external forces be such as to tend to increase the opening between surfaces 12 and 14 above the designed opening or gap, the said forces of the fluid will urge sealing ring 13 to the left, as viewed in FIG. 1, to reduce the gap to the designed size.
Whenever the pressure of the fluid is below that for which the seal is designed to operate as a gap seal, and the sealing members are rotating relative to one another, sealing ring 13 will contact mating ring 10 and thereby establish frictional contact between surfaces 12 and 14. This contact is augmented by spring 20, the function of which is to close the gap between surfaces 12 and 14 when the equipment is not operating and thereby prevent a leakage of the fluid along shaft 12 into opening 16 and also to prevent dirt particles and other harmful substances from getting between the seal surfaces 12 and 14. Although such contact is desirable when there is no relative rotation between the mating ring 10 and sealing ring 13, it is however, highly undesirable as the relative speeds and pressures between surfaces 12 and 14 increase to the designed speeds and pressures, since even during the brief period that the equipment is getting up to speed or slowing down to stop, sufficient friction and heat can be generated to destroy the surfaces 12 and 14, particularly if the fluid sealed has low lubricating qualities such as a gas.
Undesirable friction and heat are eliminated in conventional gap seals by providing shallow spiral grooves in one of the surfaces 12 or 14. The shape of the grooves is such as to cause fluid in chamber 18 to be forced radially inwardly even at relatively slow speeds of rotation of rotor 16, across the inner regions of surfaces 12 and 14. A hydrodynamic wedge is thus created which provides sufficient pressure to separate the surfaces 12 and 14 and forms a film of the fluid being sealed on which the surface 12 rides. This, in turn, eliminates or prevents, any direct contact between surfaces 12 and 14 and prevents the generation of destructive friction and heat.
Referring to FIG. 2, the spiral grooves are shown at 22. The precise shape and size of the grooves depends largely upon degree of effectiveness required of them. In the form shown in FIG. 2, they extend spirally inward across slightly more than one-half the surface 14 and terminate at 24. They should not of course, extend across the entire surface 14 since they would then provide a leak path across the seal. The area of the grooves illustrated is a little less than one-fourth the area of said surface. The area between the grooves is indicated at 23. The groove depth, area, helix angle and the distance at which the grooves terminate may be varied to suit different operating conditions. The depth of grooves 22 is preferably two or three times the actual minimum clearance or gap between surfaces 12 and 14 when the seal is in operation.
The shallow grooves 22 may be formed in any known way. Etching is the most typical.
Since the grooves 22 are spiraled, the relative direction of rotation between the surfaces 12 and 14 must be such as to cause the fluid to be forced radially inwardly through the grooves 22. This means that the surface 12 must rotate in the same direction as the direction in which the grooves 22 are spiraled. This, in turn, limits the use of the seal to an installation in which the shaft is rotating in the direction for which the seal is designed. Such limitation, however, can be eliminated by the known constructions, which employ two or more sealing rings or by using a symmetrical groove formation as discussed below.
FIG. 3 shows an alternative groove formation known in the art. More specifically, FIG. 3 shows the surface of a ring having a series of T-shaped grooves 122 formed therein. The T-shaped grooves are symmetrically disposed across the surface of the ring. The grooves 122 function in essentially the same manner as the grooves 22 shown in FIG. 2. Specifically, grooves function to cause hydrodynamic pumping effects so as to cause separation of the opposed sealing faces. As with the spiral grooves shown in FIG. 2, the T-shaped groove should not extend across the entire ring surface. One advantage of the groove configuration shown in FIG. 3 is that it is symmetrical so that it operates in the same way regardless of the direction of rotation. Thus, this type of ring formation can be used for bi-directional sealing. Like the grooves shown in FIG. 2, the T-shaped grooves shown in FIG. 3 are extremely shallow and typically formed by etching or some other relatively sophisticated.
Another known design is shown in FIG. 4. In accordance with this design, a circumferentially spaced series of tapered lands 222 are formed along the outer periphery of one of the sealing ring and the mating ring. The land is tapered such that it gradually recedes from the surface. At the more recessed end, a step-down is formed to form a sharply recessed portion 223. Thus, with reference to FIG. 4, the land tapers from the left downward toward the right with a drop off at the recess 223. Because of the non-symmetrical nature of this tapered land, this type of groove formation is not suitable for hi-directional operation. Again, however, bi-directional operation can be provided by using two similar rings as is known in the art.
Typically tapered lands of the type shown in FIG. 4 must be provided by precision machining on the smooth face of either the sealing ring or the mating ring. It is easy to appreciate that such precise machining is difficult and expensive. In operation the ring formation shown in FIG. 4 operates in essentially the same way as the ring configuration shown in FIG. 2 and FIG. 3. In particular, the surface formation causes a pressurization of the fluid between the sealing faces causing a radial gap to form between the sealing faces.
The addition of spiral, T-shaped grooves or tapered lands provides hydrodynamic load support for the sealing ring 13. Upon the start of rotation, fluid is pumped between the faces of the seal, and at a given RPM, the hydrodynamic load support becomes sufficient to give complete separation. The seal is operable at zero pressure because of the spring force pushing the surfaces together.
It is understood that the grooves may be formed in surface 12 of mating ring 10 in FIG. 1, instead of in the confronting surface on the sealing ring 13. It is also understood that the curvature, if desired, may be formed on the surface or surfaces of the mating ring with the grooved sealing ring having a flat surface. It is also possible to eliminate the curvature especially if, as with the present invention, another way of achieving the desired effect is provided.
The present invention is intended to replace known designs in which the surface of the mating ring is etched to create surface grooves which create gas dynamic effects. In these known designs, the groove is very shallow--on the order of millionths of an inch deep. Typically, the grooves are formed by a photoetch process which is complicated and expensive. Moreover, even with these extremely shallow grooves, there is a step at the transition between the groove and the surface in which the groove is formed. This step tends to create non-laminar flow of the sealing fluid. It is known that the best sealing effects are achieved when the laminar flow of the sealing fluid is maintained.
There remains a need in the art to have a controllable mechanical seal where the thickness of the lubricating fluid film can be maintained at a practical thickness and one in which the manufacturing tolerances are reduced. It is further desirable to have a seal arrangement where contaminants are less likely to impact upon the performance of the seal, and, one in which the seal can self adjust for any shaft misalignment. Further, it is desirable to have a seal which will operate over a broader range and reduce any ultimate seal wear by obtaining an optimum film thickness over a wide range of operating conditions.
One improved mechanical face seal is disclosed in the present inventor's previous U.S. Pat. No. 4,738,453. In that patent, a controllable mechanical seal was disclosed for a machine having a housing and a shaft that is rotatable relative to the housing. The seal includes a stationary cylindrical seat member and a rotatable cylindrical nose piece. The nose piece is fitted with a plurality of lift pads that are held by the nose piece and the nose piece is also fitted with a fluid dam which defines a radial face surface. One or the other of the parts, either the seat, that is, or the nose piece will be rotatable with the shaft and suitable means will bias one element toward the other. The lift pads are particularly formed as stool like units, having flexible leg ligaments that extend at an angle to the pad face, so that the pad face may move in up to three degrees of freedom to form a fluid film between the pad face and the seat member to adjust for shaft misalignment and to provide equal loading among the lift pads. Equal loading among pads in the longitudinal shaft axis direction is provided by dog leg type bends in the ligament construction.
In any mechanical face seal, it is important that the two rings, the mating ring and the sealing ring are aligned such that their faces are in flush contact. Often, this is done by making one of the rings floating and spring biasing the two rings together so that the two rings are pressed into flush contact by the springs.
Many known designs employ complicated expensive alignment, mechanisms or arrangements. There is still a need for a simple inexpensive way to align the sealing faces relative to one another. There is also a need for a more simple spring biasing construction.
In any gap seal the sealing ring and the mating ring must rotate about concentric axes to ensure proper performance. Since the mating ring typically rotates with the shaft and the sealing ring is secured in the housing, the shaft must be supported for rotation about a fixed axis in the housing in order for the gap seal assembly to function properly. In other words, the eccentricity of the shaft must be minimized to avoid shaft runout.
In the past this has meant that ball bearings must be used to support the shaft for rotation about a fixed axis. Ball bearings are far from ideal. They tend to wear rapidly at high speeds and are expensive for that reason. Conventional hydrodynamic bearings cannot be used, however, because the shaft position is not fixed until the shaft reaches design speed. The eccentricity of the shaft during the start-up would lead to undesirable movement of the sealing faces which would defeat the gap seal. There remains a need for a durable inexpensive bearing for supporting the shaft in a gap seal assembly for rotation about a fixed axis.